Currents may need to be measured, for example when controlling a load current or to provide over-current protection. The current may be measured by measuring another proportional current. The proportionality factor of the currents may be given by a resistance ratio. The accuracy of the measurement may depend on the accuracy of the resistance ratio. However, the resistance ratio may change over time and temperature and may be subject to manufacturing variations. The overall uncertainty may be as much as 28%.
The resistance ratio may be corrected after end of line testing. However, this usually involves additional costs and not all production technologies offer correction factors. Further, the use of correction factors will not solve the problems of drift over time and temperature.